The boatswain's or bosuns whistle

The boatswain's whistle (pronounced bosun's whistle) is a small whistle 5 inch long, that was used aboard old sailing ships since the 15th century, usually to gain the attention of the crew, also to welcome aboard visitors. Historically the boatswain's call was used to pass commands to the crew when the voice could not be heard over the sounds of the sea. Because of its high pitch, it could be heard over the activities of the crew and bad weather. The pipe consists of a narrow tube (the gun) which directs air over a metal sphere (the buoy) with a hole in the top. The player opens and closes the hand over the hole to change the pitch. The rest of the pipe consists of a "keel", a flat piece of metal beneath the gun that holds the call together, and the "shackle", a keyring that connects a long silver or brass chain that sits around the collar, when in ceremonial uniform.